Test fixtures are commonly employed for the electrical testing of printed circuit boards and integrated circuit chips. In the case of printed circuit boards, the completed printed circuit board is placed on the test fixture; and electrical connections are made to selected contact points on the printed circuit board for effecting the electrical tests. In the case of integrated circuits, the integrated circuit typically is mounted on a printed circuit wiring board, which then is used in the test fixture to complete electrical circuit interconnections through the wiring board to the test fixture.
In both printed circuit board test fixtures and integrated circuit test fixtures of the type where the integrated circuit chip is placed on a board, it is very important that accurate registration is effected between probes in the test fixture and corresponding circuit connection points on the test boards. The registration between the probes and the contact points on the test boards necessarily must be capable of repetition over a large number of cycles of operation of the test apparatus.
Typically, the probes in the test apparatus constitute telescoping spring biased probe connectors, which exert a predetermined force against the contact points or positions on the underside of the printed circuit board which is undergoing test, or which is used to operate as an interface to an integrated circuit undergoing test. To initiate a test, the circuit board is pressed downwardly onto the test fixture to depress the probes which make contacts with appropriate electrical contact points on the underside of the circuit board. When the circuit board is removed, the probes extend to their maximum extension length, which necessarily is above the upper planar surface of the test fixture itself. If the probes are left in this exposed position, it is possible for the probes to become damaged accidentally, or, if a test fixture is not properly aligned with the probes, to cause bending or damage of the probes as the circuit board under test is depressed or moved downwardly onto the test fixture.
To overcome the potential for damaging the relatively small and delicate spring loaded probes, test fixtures have been developed in which a movable plate, having aligned holes or apertures in it, is mounted for reciprocal movement above the top of the portion of the test fixture to which the probes are attached. When the movable plate is biased upwardly out of engagement with the surface of the test fixture, or is biased to its uppermost position, the tips of the probes either are withdrawn completely from the movable plate or extend part way into holes in the movable plate. The probes, however, do not extend above the upper surface of the movable plate. When such devices are used, the integrated circuit board for the device under test then is placed and located on top of the movable plate. Once the circuit board is in place, the movable or floating plate is lowered. This causes the probes or pins to extend through the holes in the movable plate to make electrical contact with the circuit board, which is carried on the movable or floating plate. In many devices of this type, a vacuum is used to draw the floating plate and the circuit board downwardly until various ones of the spring pin probes make contact through the apertures in the floating plate with the board under test. In some cases, the movement of the board and floating plate downwardly is effected manually, without a vacuum, by means of cams or other mechanical apparatus. The electrical operation of the devices, however, are the same in either case.
The Murray U.S. Pat. No. 4,544,886; Fowler U.S. Pat. No. 4,573,009; Angelucci U.S. Pat. No. 4,841,231; Seavey U.S. Pat. No. 5,247,249; Metrone U.S. Pat. No. 4,321,533; and Nesbitt U.S. Pat. No. 4,746,861 are directed to the overall technique described above, employing a floating plate for effecting accurate registration between test probes or spring pin connectors. All of these patents employ the underlying idea of a base plate with spring probes or spring pin connectors soldered or otherwise attached to the base plate. A spring biased floating plate is placed above the base plate. This floating plate has apertures through it, which permit the probes or spring pin connectors to extend through the apertures. In the absence of any loading, the tips of the pins either are below these apertures or are located within them.
The Janko U.S. Pat. No. 5,015,946 somewhat reverses the process of the patents noted above, and operates to lower the spring loaded probes onto the device under test. The overall operation of the device of Janko, however, is similar to that of the other patents where the printed circuit board and floating plate are drawn downwardly by the vacuum supply or other mechanical apparatus.
In all of the patents noted above, the base plate and the floating or movable plate are made of insulating material to avoid any electrical shorts between these plates and the connector probes and the circuit board connectors placed on top of the floating plate. Thus, if high frequency signals are passed through the probes or spring pin connectors, interference between at least adjacent probes may take place, resulting in erroneous test results.
It is desirable to provide a simple, spring-loaded, high frequency interface system for a test fixture, which provides the protection of the prior art devices noted above for the probes or spring pin connectors, and which further provides electrical shielding for the probes.